My advice would be to talk to Elderly Instruments in Lansing, Michigan. They sell several banjos equipped with nylons and should know the guages and lengths.
DaveO - My sense is that the nylons were/are tuned about two steps lower than the standard gDGBD tuning for steel strings (in an open E??).
Mousethief - from listening to a group called Dr. Horsehair on a CD titled "Wizard Oil," the nylon/gut strings have a softer, mellower, klunkier sound that is quite attractive. Bob Flescher (sp?) who is playing banjo on the CD has won at Galax several times and is an expert. He is playing with bass, guitar, bones and tambourine - and is using a Civil War era banjo. It's worth a listen.